It has long been desired to fabricate various airframe components, such as canards, winglets, or wings, as hollow bodies. The benefits of using hollow bodies for these airframe components include a substantial reduction in weight, which results in improved fuel efficiency and increased performance.
In a number of applications, an airframe component, such as the leading edge or wing tip of an aircraft's wing, is required to have a shape that would, if made using conventional fabrication techniques, require multiple part fabrication and assembly procedures. For example, one such airfoil structure typically used as a component in the manufacture of aircraft wings is a hollow airfoil shell having a complex, curved leading edge and an open trailing section. This airfoil structure is conventionally fabricated from several parts, which are assembled, joined together with a wing tip section and a trailing edge section, and then partially inserted into and attached to the aircraft fuselage to form the aircraft wing.
Conventionally, these hollow airfoil structures have been constructed using a systematic build-up of multiple subcomponent assemblies. The individual sheet assemblies have comprised a mixture of product forms. These forms included, for example, extruded, forged, cast, or formed sheet, which were then mechanically fastened and tied to the other subcomponent assemblies to fabricate the airfoil structure. At the part level, individual parts required processing through the fabrication stage, then fastened with mating structures during the assembly stage of the manufacturing operation.
With the introduction of new material systems in airframe design, efficient methods to fabricate and integrate subassemblies made from different materials, such as metallic, non-metallic, and matrix elements, have become increasingly important, for example, for improving the weight, cost, and life expectancy of the resultant aircraft structure. A continuing desire of those skilled in the art is to develop structures and methods for forming such structures to significantly reduce the total number of structures and steps required for the final assembly.
Many of the methods developed to manufacture hollow airfoil structures utilize superplastic forming ("SPF") techniques, which techniques rely on the capability of certain materials, such as titanium alloys, to develop unusually high tensile elongation with a minimal tendency towards necking when subjected to coordinated time-temperature-strain conditions within a limited range. This characteristic has been known in the art and used in producing of a wide variety of strong, lightweight metallic structures.
One prior art method involves forming a closed cellular structure from two or more separate layers of sheet material. The two or more layers are joined along respective edge portions (e.g., by welding or diffusion bonding) to form an inflatable envelope assembly. This inflatable assembly is then superplastically formed to produce an integral structural part having a predetermined shape.
It is known to insert one or more inlet tubes between the sheets that comprise an envelope assembly to supply gas under pressure to the interior of the envelope assembly to form the assembly into the desired shape using superplastic forming. The gas supply tube is first positioned and the envelope assembly is then sealed around its periphery to form a gas-tight structure. This sealing typically requires labor intensive and expensive methods, such as seam welding, partial penetration welding, or diffusion bonding using heat and pressure.
It is also known to form an airfoil using a single sheet. The single sheet is formed into a folded over sandwich structure and sealed along its periphery by a continuous weld to form an expandable envelope. This envelope structure is placed in a limiting structure, such as a containment die, and a gas is injected into the interior portion of the envelope structure under superplastic conditions to form or expand the single sheet. Such expansion may occur in two opposing directions. Thus, by applying appropriate internal pressure and temperature to the envelope structure, the envelope may be expanded into the surrounding die configuration, thereby producing the desired structural part.
These prior art techniques suffer from significant disadvantages in addition to those previously mentioned. For example, they require welding or diffusion bonding the periphery of the sheet assembly prior to superplastic forming and then cutting off the welded areas around the periphery of the expanded assembly to form the shell structure. Such removal is labor intensive and inefficient. The welded seam must be removed so that one or more internal members (e.g., internal reinforcing members) can be easily put into position. For example, if the welded seam is not removed from, e.g., the trailing edge, it will be impossible to spread the two major faces of the expanded assembly apart sufficiently to allow an internal member to be positioned.
Additionally, with prior art techniques there is a risk that the one or more inlet tubes that are positioned to supply the gas to the interior of the expandable envelope or sheet assembly will be pinched closed and rendered inoperable while welding the sandwich assembly. The inlet tubes are typically discarded after each use, which increases costs.
Accordingly, there is a continuing need for a method that can be used to create large structural components with reduced manufacturing and assembly costs, reduced part count (i.e., fewer parts), and reduced fasteners, which method results in a structural component having a less complex overall structure. Such a method could desirably use a single sheet that is superplastically formed in two opposing directions, without the need to weld or diffusion bond the edges of the sheet together. Desirably, in making an airfoil structure, such a method would eliminate the need to weld leading and/or trailing edge structures or on aerodynamic centerlines, thus resulting in an airfoil structure having a continuous leading edge. The elimination of welding or diffusion bonding would also enable the method to work with a wider variety of materials, such as aluminum. It would also be desirable to be able to form structures from a sheet without the special preparation of the sheet that is now needed to allow the sheet to accept and hold the gas inlet tubes.